CORAL AGES AND ISLAND SUBSIDENCE, HILO DRILL HOLE

A 25.8-m-thick sedimentary section containing coral fragments occurs d irectly below a surface lava flow (the similar to 1340 pear old Panaew a lava flow) at the Hilo drill hole. Ten coral samples from this secti on dated by accelerator mass spectrometry (AMS) radiocarbon and five b y thermal infrared multispectral scanner (TIMS) Th-230/U methods show good agreement. The calcareous unit is 9790 years old at the bottom an d 1690 years old at the top and was deposited in a shallow lagoon behi nd an actively growing reef. This sedimentary unit is underlain by a 3 4-m-thick lava flow which in turn overlies a thin volcaniclastic silt with coral fragments that yield a single C-14 date of 10,340 years. Th e age-depth relations of the dated samples can be compared with propos ed eustatic sea level curves after allowance for island subsidence is taken. Island subsidence averages 2.2 mm/yr for the last 47 years base d on measurements from a tide gage near the drill hole or 2.5-2.6 mm/y r for the last 500,000 years based on the ages and depths of a series of drowned coral reefs offshore from west Hawaii. The age-depth measur ements of coral fragments are more consistent with eustatic sea levels as determined by coral dating at Barbados and Albrolhos Islands than those based on oxygen isotopic data from deep sea cores. The Panaewa l ava flow entered a lagoon underlain by coral debris and covered the dr ill site with 30.9 m of lava of which 11 m was above sea level, This s urface has: now subsided to 4.2 m above sea level, but it demonstrates how a modern lava flow entering Hilo Bay would not only change the co astline but could extensively modify the offshore shelf.